of the Light emitted by Luminous Bodies. 15 



The method of ascertaining when the lights are at 

 the proper height has already been described. 



In order that the two long and narrow tables or 

 platforms just described, upon which the -lights move, 

 may remain immovable in their proper positions, they 

 are both firmly fixed to a very strong stand which 

 supports the photometer; and in order that the mo- 

 tion of the carriages which carry the lights may be as 

 soft and gentle as possible, they are made to slide 

 upon parallel brass wires, 9 inches asunder, about 

 IT}- of an inch in diameter, and well polished, which 

 are stretched out upon the tables from one end to the 

 other. (See Plate III.) 



The pane of glass covered with white paper, which, 

 being fixed in a groove in the back of the box, consti- 

 tutes the vertical plane upon which the shadows are 

 projected, is 5^ inches long and 3! inches wide, as has 

 already been observed, which is much larger than 

 the dimensions assigned above for the field ; namely, 

 1 1*0 inches wide and 2 inches high. I had two ob- 

 jects in view in this arrangement : first, to render it 

 easier to fix this plane in its proper position ; and, 

 secondly, to be able to augment occasionally the di- 

 mensions of the field, by removing entirely the black 

 pasteboard screen from before this plane, or making 

 use of another with a larger aperture, which is some- 

 times advantageous.* 



* Since writing the above, I have made a little alteration in the form of the 

 box which contains my photometer. The front of it, instead of being open, is 

 now closed ; and the light is admitted through two horizontal tubes, which are 

 placed so as to form an angle of 60, their axes meeting at the centre of the 

 field of the instrument. (See Fig. I, Plate I.) The field of the photometer is 

 viewed through an opening made for that purpose in the middle of the front of 

 the box, between the two tubes above mentioned. The Plates 1., II., III., and 

 IV. will serve to give a clearer idea of the instrument, in its present most im- 

 proved state. 



